When connecting to the Nematron OpenContol
OPC Server you will receive an error when the Items (Tags) are
added. Also the Server will most likely crash.

Solution:

This is a problem in the OpenControl
Server. Whenever items are added it will return an incorrect
error and crash whenever a requested data type is specified, even
when it matches the server type. It will only work when it is
requested to use the native data type. To make the Citect OPC
Client driver work you have to change the request to use the native
data type.

This workaround can be enabled by the Citect.ini
parameter...

[OPC]
AddItemAsVtEmpty=1

Only use the above workaround when absolutely necessary. As
explained in KB article Q2883

The above workaround is enabled in the OPC Client driver shipped
with Citect for Windows 5.21.